Piezoelectric material is material which converts mechanical or electrical energy to electrical or mechanical energy. A typical example of the piezoelectric material is an oxide having a perovskite crystal structure such as lead zirconate titanate (Pb(Zr, Ti)O3, hereinafter referred to as PZT). Regarding the ratio between Zr and Ti Zr/Ti=53/47 at % as a threshold, perovskite PZT becomes rhombohedral when the ratio of Zr is high, or tetragonal when the ratio of Zr is low. The rhombohedral PZT gives the maximum piezoelectric displacement along the (111) axis, while the tetragonal PZT gives the maximum piezoelectric displacement along the (001) axis (c-axis). However, most of the piezoelectric material are polycrystals constituted of aggregates of crystal grains and the crystallographic axes of the crystal grains are oriented toward various directions. Therefore, the directions of spontaneous polarization Ps are also oriented to different directions.
In accordance with downsizing of electronic devices in recent years, there is a growing demand for downsizing of piezoelectric elements. In order to meet the demand, there is a shift toward the use of thin-film piezoelectric elements having remarkably smaller volume than conventional piezoelectric elements in the form of a sintered body.
Therefore, researches and development have been actively carried out for the purpose of thinning the piezoelectric elements.
In general, a thin piezoelectric film made of PZT-based piezoelectric material is likely to have orientation along the (111) plane. However, orientations along the other surfaces are also recognized because the degree of the (111) orientation is low. Therefore, the thin piezoelectric film does not have enough piezoelectric property to function as a piezoelectric element and the property variations are also remarkable.
For this reason, through refinement of substrates and electrodes, thin piezoelectric films having orientation along the (001) or (100) plane have been manufactured as described below.
For example, rhombohedral PZT shows spontaneous polarization Ps along the (111) axis direction, while tetragonal PZT shows spontaneous polarization Ps along the (001) axis direction. Therefore, in order to achieve high piezoelectric property even when the piezoelectric element is thinned down, the (111) axis of the rhombohedral PZT or the (001) axis of the tetragonal PZT needs to be oriented perpendicular to the surface of the substrate. Conventionally, in order to achieve almost 100% degree of the orientation in tetragonal perovskite PZT, a thin, highly crystalline PZT film is formed directly on a monocrystalline magnesium oxide (MgO) substrate having a rock salt structure sliced to have the (100) crystal orientation plane at the top surface thereof by sputtering using a tetragonal PZT target at a temperature of 600 to 700° C. The thin PZT film has orientation along the (001) axis which is perpendicular to the surface of the substrate. (e.g., see Japanese Unexamined Patent Publication No. H 10-209517 and Journal of Applied Physics, U.S.A., the American Institute of Physics, Feb. 15, 1989, Vol. 65, No. 4, pp. 1666-1670). In this case, a 0.1 μm thick piezoelectric layer which is made of PbTiO3 or (Pb, La)TiO3 and free from Zr is formed on a Pt electrode oriented along the (100) plane as a base layer for the thin PZT film. Then, a thin PZT film of 2.5 μm in thickness is formed thereon by sputtering. By so doing, a low crystalline Zr oxide layer is less likely to be formed in the early stage of the formation of the thin PZT film. As a result, the thin PZT film is obtained with higher crystallinity. Specifically, the thin PZT film is obtained with the degree of orientation along the (001) plane (α(001)) which is almost 100%. The degree of orientation α(001) is defined byα(001)=I(001)/ΣI(hkl)
ΣI(hkl) is the sum total of diffraction peak intensities from the crystal planes of perovskite PZT which are obtained by X-ray diffraction using Cu—Kα rays where 2θ is 10 to 70°. The (002) and (200) planes are not included in the value ΣI(hkl) because they are equivalent to the planes (001) and (100).
In this case, however, since the monocrystalline MgO substrate is used as a base substrate, the obtained piezoelectric element will be expensive and an inkjet head using the piezoelectric element will also be expensive. Moreover, the substrate material is disadvantageously limited to monocrystalline MgO only.
Therefore, various methods have been developed as described below to form a film having crystal orientation along the (001) or (100) plane using perovskite piezoelectric material such as PZT on an inexpensive substrate made of silicon or other material.
For example, Japanese Patent Gazette No. 3021930 discloses that a PZT film having preferred orientation along the (100) plane can be realized by applying a PZT precursor solution containing PZT or lanthanum-containing PZT on a Pt electrode oriented along the (111) plane, thermally decomposing the precursor solution at 150 to 550° C. and then heating the thermally decomposed precursor solution at 550 to 800° C. for crystallization (sol-gel method).
Further, according to the method disclosed by Japanese Unexamined Patent Publication No. 2001-88294, an ultrathin titanium layer is formed on a bottom electrode made of iridium such that the crystal orientation of a PZT film to be formed thereon is controlled. In this method, a base layer based on zirconium oxide is formed on a substrate made of silicon or other material, a bottom electrode containing iridium is formed on the base layer and an ultrathin titanium layer is formed on the bottom electrode. Then, a thin amorphous film containing a metal element and an oxygen element is formed thereon as a precursor of a thin piezoelectric film and heated at a high temperature for crystallization (sol-gel method). Thus, a thin perovskite piezoelectric film is obtained. According to this method, the crystal orientation of the thin piezoelectric film such as PZT is controlled by the thickness of the titanium layer. If the thickness of the titanium layer is set to 10 to 20 nm, the thin piezoelectric film is oriented along the (111) plane.
According to Japanese Unexamined Patent Publication No. H11-191646, a thin piezoelectric film is formed by a sol-gel method. In this method, a titanium layer of 4 to 6 nm in thickness is formed on a Pt electrode oriented along the (111) plane such that titanium oxide derived from titanium in the titanium layer is used as a crystal seed to obtain a PZT film oriented along the (100) plane.
On the other hand, according to Japanese Unexamined Patent Publication No. 2000-208828 (pp. 3-4), a sol containing Zr and Ti in the concentration ratio of Zr/Ti=75/25 is applied by spin-coating on a RuO2 bottom electrode formed by sputtering on a SrTiO3 substrate and dried by heating to form a precursor film. Then, several precursor films are formed thereon by using a sol containing Zr and Ti in the concentration ratio of Zr/Ti=52/48. Then, the resulting product is annealed at a high temperature of 900° C. to obtain a thin film of PZT-based piezoelectric oxide having a columnar structure and (001) crystal orientation without causing cracks.
All the above-described methods are advantageous in that the expensive monocrystalline MgO substrate is not used. However, unlike the method of forming the thin piezoelectric film on the monocrystalline MgO substrate, it is difficult to obtain a thin, highly crystalline piezoelectric film having crystal orientation defined at the time when the film has been formed because the sol-gel method is used. Therefore, a thin amorphous piezoelectric film is formed first and then a layered film including the thin piezoelectric film is subjected to heating together with the substrate so that the crystallographic axis has preferred orientation along an appropriate direction.
If mass production of the piezoelectric element is carried out by the sol-gel method, a thin amorphous precursor film of the thin piezoelectric film is likely to cause cracks due to a change in volume in a degreasing step for removing organic matters. Further, in the step of heating the thin amorphous precursor film of the thin piezoelectric film at a high temperature for crystallization, cracks are easily generated due to a change in crystallinity and the film is likely to fall off the bottom electrode.
Further, in the sol-gel method, the thickness of the PZT film obtained in a single step (application of a precursor solution and the following heat treatment) is about 100 nm. Therefore, in order to achieve a thickness of 1 μm or more required for achieving the piezoelectric element, the step needs to be carried out 10 or more times. This brings about a problem of reduction of yield.
As a solution of these problems involved in the sol-gel method, Japanese Unexamined Patent Publications Nos. 2000-252544 and H10-81016 disclose an advantage of adding titanium or titanium oxide to the bottom electrode. In particular, Japanese Unexamined Patent Publication No. H10-81016 discloses that a PZT film oriented along the (100) plane is achieved by sputtering.
However, in fact, a perovskite PZT film is not directly formed on the bottom electrode. Instead, an amorphous or pyrochlore PZT film is formed at a low temperature of 200° C. or lower and then the PZT film is heated in an oxygen atmosphere at a high temperature of 500 to 700° C. for crystallization. Therefore, like the sol-gel method, cracks are easily generated or the film is likely to fall off the bottom electrode due to a change in crystallinity in the high-temperature heating step for crystallization. Further, in any of the above-described methods, the degree of (001) or (100) orientation of the PZT film formed by the sol-gel method is 85% or less.
On the other hand, in Japanese Unexamined Patent Publication No. 2001-88294, an attempt has been made to form an orientation-controlled PZT film on an ultrathin titanium layer formed on the surface of a Ir bottom electrode by other methods than the sol-gel method (including MOD method) of forming a thin amorphous film first and then heating the film to turn into a thin crystalline film, i.e., methods which achieve direct formation of a thin crystalline film without heat treatment for crystallization such as sputtering, laser abrasion and CVD. However, the orientation film has not been achieved by the other methods than the sol-gel method. As a reason for the above, it is considered that the sol-gel method achieves the crystallization of the PZT film from the bottom electrode side to the top electrode side, whereas by CVD or sputtering, the crystallization progresses at random without regularity. Therefore, the crystallization is hard to control in these methods.
Further, Japanese Patent Gazette No. 3481235 discloses a method which does not require a post annealing step. According to the method, a thin electrode film made of an alloy of noble metal such as platinum or iridium containing titanium is formed as a bottom electrode by sputtering, a thin oxide film which is made of perovskite lead lanthanum titanate (PLT) free from Zr in its composition and oriented along the (001) plane is formed as an initial layer by sputtering and then a thin PZT film is formed thereon using the PLT film as a base, thereby obtaining a thin PZT film oriented along the (001) plane. Moreover, Japanese Unexamined Patent Publication No. 2004-79991 discloses that use of a thin electrode film made of a noble metal alloy containing cobalt, nickel, manganese, iron or copper makes it possible to form a PZT film oriented along the (001) plane directly on the thin electrode film. As described above, if the PZT film having (001) crystal orientation which exhibits a high piezoelectric constant is achieved, a thin piezoelectric film having high piezoelectric property is obtained. The thin piezoelectric film has (001) preferred orientation perpendicular to the surface of the substrate. If the thin piezoelectric film has a tetragonal perovskite structure, the polarization is oriented along the (001) plane, i.e., the same direction as the preferred crystal orientation. Therefore, high piezoelectric property is exhibited. For these reasons, the thin piezoelectric film described above is expected in various fields as an actuator which achieves large displacement with a small applied voltage.